Introduction to Lesson 4 - The Lymphatic System Your circulatory system is not your body s only vascular transport system. Closely associated with the blood vessels of the circulatory system is the lymphatic system. The lymphatic system is a network of glands and vessels that extend throughout your body. Lesson 4 Outcomes By the end of this lesson, you should be able to: Describe the function of the lymphatic system in the human body. List the components of lymph in the human body, i.e., fat, protein, water, white blood cells. Identify the following structures of the lymphatic system from a specimen, model, or diagram: adenoids tonsil lymph nodes spleen thoracic duct Differentiate between lymph vessels and blood vessels. Lesson 4 Overview Following is a list of topics covered in this lesson. Function of the Lymphatic System Lymph 1
Lymphatic Organs The Lymphatic System The lymphatic system consists of fluid called lymph, vessels that transport the lymph and organs that contain lymphoid tissue. The lymphatic system has three primary functions. First of all, it returns excess interstitial fluid (the fluid that surrounds the cells) to the blood. Interstitial fluid is very abundant, making up about 15% of the body mass. It is the clear, colourless liquid that appears when the skin is grazed or a blister is broken. All of the cells and tissues of the body must be continuously bathed by fluids. These fluids enable nutrients to pass from the capillaries to the cell membranes and waste products to return to the capillaries. Some of the fluid in blood is constantly passing through the capillary walls and entering the spaces between the cells. The walls of the capillaries prevent most of the blood cells and some of the proteins of the plasma from leaving the blood stream. However, some white blood cells leave the capillaries by forcing their way out between the cells that make up the capillary walls. The composition of intercellular fluid is much the same as the plasma of the blood. Interstitial fluid bridges the gap between capillaries and the isolated cells which are not in direct contact with a capillary. It is laden with substances needed by the cells and also takes away waste substances from the cells. Of the fluid that leaves the capillaries, about 90 percent is returned. The 10 percent that does not return becomes part of the interstitial fluid that surrounds the tissue cells. Lymph capillaries pick up the excess interstitial fluid and proteins and return them to the venous blood. The second and probably most well known function of the lymphatic system is defense against invading microorganisms and disease. Lymph nodes and other lymphatic organs filter the lymph to remove microorganisms and other foreign particles. Lymphatic organs also contain lymphocytes that destroy invading organisms. Lymphatic organs will be discussed later in the lesson. The third function of the lymphatic system is the absorption of fats and fatsoluble vitamins from the digestive system and the subsequent transport of these substances to the venous circulation. The lining of the small intestine is covered with fingerlike projections called villi. There are blood capillaries and special lymph 2
capillaries, called lacteals, in the center of each villus. The blood capillaries absorb most nutrients, but the fats and fat-soluble vitamins are absorbed by the lacteals. The absorbed fat droplets are transported to adipose (fat) tissue where they are stored. Figure 5.4.3 Lacteal (http://users.rcn.com/jkimball.ma.ultranet/biologypages/g/gitract.html#intestine) Lymph Lymph is a fluid similar in composition to blood plasma (90% water, salts, proteins, hormones, nutrients, waste products, gases). As discussed above, it is derived from blood plasma as fluids pass through capillary walls at the arterial end. As the interstitial fluid begins to accumulate, it is picked up and removed by tiny lymphatic vessels and returned to the blood. As soon as the interstitial fluid enters the lymph capillaries, it is called lymph. Returning the fluid to the blood prevents a condition 3
known as edema (swelling of the tissue) and helps to maintain normal blood volume and pressure. The lymphatic system consists of capillaries and lymph vessels that correspond to the capillaries and veins of the blood circulatory system. Lymphatic vessels, unlike blood vessels, only carry fluid away from the tissues. The smallest lymphatic vessels are the lymph capillaries, which begin in the tissue spaces as blind-ended sacs. Lymph capillaries are found in all regions of the body except the bone marrow, central nervous system, and tissues, such as the epidermis, that lack blood vessels. The wall of the lymph capillary is composed of endothelium in which the simple squamous cells overlap to form a simple one-way valve. This arrangement permits fluid to enter the capillary but prevents lymph from leaving the vessel. The lymph capillaries do not form a net like the blood capillaries but they resemble microscopic fingers that connect with the lymph vessels. The smaller lymph vessels unite, forming larger ones. See the diagram below. Figure 5.4.1 Lymph Capillaries (http://training.seer.cancer.gov/module_anatomy/unit8_2_lymph_compo.html) Small lymphatic vessels join to form larger tributaries, called lymphatic trunks, which drain large regions. Lymphatic trunks merge until the lymph enters the two lymphatic ducts. The right lymphatic duct drains lymph from the upper right quadrant of the body. The thoracic duct drains all the rest. The diagram below illustrates the major lymphatic vessels and organs. 4
Figure 5.4.2 Lymphatic Vessels and Organs (http://www.acm.uiuc.edu/sigbio/project/lymphatic/index.html) Like veins, the lymphatic tributaries have thin walls and have valves to prevent backflow of blood. There is no pump in the lymphatic system like the heart in the cardiovascular system. The pressure gradients to move lymph through the vessels come from the skeletal muscle action, respiratory movement, and contraction of smooth muscle in vessel walls. Lymphatic Organs Lymphatic organs are characterized by clusters of lymphocytes and other white blood cells, such as macrophages in a framework of short, branching connective tissue fibers. The lymphocytes originate in the red bone marrow with other types of blood cells and are carried in the blood from the bone marrow to the lymphatic organs. When the body 5
is exposed to microorganisms and other foreign substances, the lymphocytes multiply within the lymphatic organs and are sent in the blood to the site of the invasion. This is part of the immune response that attempts to destroy the invading agent. Lymph nodes are small bean-shaped structures that are usually less than 2.5 cm in length. They are widely distributed throughout the body along the lymphatic pathways where they filter the lymph before it is returned to the blood. There are three regions on each side of the body where lymph nodes tend to cluster. These areas are the groin, the armpit, and the neck. The typical lymph node is surrounded by a connective tissue capsule and divided into compartments called lymph nodules. The lymph nodules are dense masses of lymphocytes and macrophages and are separated by spaces called lymph sinuses. Several afferent lymphatic vessels, which carry lymph into the node, enter the node on the convex side. The lymph moves through the lymph sinuses and enters an efferent lymphatic vessel, which carries the lymph away from the node. Because there are more afferent vessels than efferent vessels, the passage of lymph through the sinuses is slowed down, which allow time for the cleansing process. See the diagram below. Figure 5.4.4 Lymph Node (http://training.seer.cancer.gov/module_anatomy/unit8_2_lymph_compo.html) Tonsils are clusters of lymphatic tissue just under the mucous membranes that line the nose, mouth, and throat (pharynx). There are three groups of tonsils. The pharyngeal tonsils are located near the opening of the nasal cavity into the pharynx. When these tonsils become enlarged they may interfere with breathing and are called adenoids. The palatine tonsils are located near the opening of the oral cavity into 6
the pharynx. Lingual tonsils are located on the rear surface of the tongue, which also places them near the opening of the oral cavity into the pharynx. Lymphocytes and macrophages in the tonsils provide protection against harmful substances and pathogens that may enter the body through the nose or mouth. The spleen is located in the upper left abdominal cavity, just beneath the diaphragm, and behind the stomach. It is similar to a lymph node in shape and structure but it is much larger. The spleen is the largest lymphatic organ in the body. Surrounded by a connective tissue capsule, which extends inward to divide the organ into lobules, the spleen consists of two types of tissue called white pulp and red pulp. The white pulp is lymphatic tissue consisting mainly of lymphocytes around arteries. The red pulp consists of venous sinuses filled with blood and cords of lymphatic cells, such as lymphocytes and macrophages. Blood enters the spleen through the splenic artery, moves through the sinuses where it is filtered, then leaves through the splenic vein. The spleen filters blood in much the way that the lymph nodes filter lymph. Lymphocytes in the spleen react to pathogens in the blood and attempt to destroy them. Macrophages then engulf the resulting debris, the damaged cells, and the other large particles. The spleen, along with the liver, removes old and damaged erythrocytes (red blood cells) from the circulating blood. Like other lymphatic tissue, it produces lymphocytes, especially in response to invading pathogens. The sinuses in the spleen are a reservoir for blood. In emergencies such as hemorrhage, smooth muscle in the vessel walls and in the capsule of the spleen contracts. This squeezes the blood out of the spleen into the general circulation. The thymus is a soft organ with two lobes that is located in front of the ascending aorta and behind the sternum. It is relatively large in infants and children but after puberty it begins to decrease in size so that in older adults it is quite small. The primary function of the thymus is the processing and maturation of special lymphocytes called T- lymphocytes or T-cells. While in the thymus, the lymphocytes do not respond to pathogens and foreign agents. After the lymphocytes have matured, they enter the blood and go to other lymphatic organs where they help provide defense against disease. The thymus also produces a hormone, thymosin, which stimulates the maturation of lymphocytes in other lymphatic organs. Lesson 4 Exercise 1. What are the three main functions of the lymphatic system? 7
2. Explain the importance of interstitial fluid. 3. a) What is the composition of lymph? b) How is lymph different from interstitial fluid? 4. a) What are lymph vessels and what is their relationship to the circulatory system? b) What are the two main lymph ducts in the body? c) How is the structure and function of lymph vessels similar to that of veins? 5. How does the lymphatic system contribute to the immune response? 6. a) Where are the main clusters of lymph nodes located? 8
b) What is the main function of lymph nodes? 7. What are adenoids? 8. Why is the spleen an important organ? 9. What role does the thymus play in immunity? Lesson 4 Summary The lymphatic system plays a vital role in the maintenance of homeostasis in the human body. This lesson has introduced you to the role of the lymphatic system in the maintenance of homeostasis in the human body, the relationship between the circulatory and lymphatic systems and the function of the lymphatic system in other body systems. The next lesson will study diseases and disorders of the circulatory system. 9